__Keywords:__microwave radiometer pulse-amplitude modulation pulse-duration modulation zero method

A.V. Filatov, A.V. Ubaychin, A.O. Сhydinov, E.I. Rozina

Modified method of zero receiving and its application in microwave radiometers was considered. This method uses combined pulse modulation. This modification bases on the original functioning algorithm of the auto regulating of zero balance follow-up system. This allows aligning the energy of signals at the radiometer receiver input to different half-periods of a symmetric pulse modulation by corresponding alignment volt-second pulse areas of modulated sequences at the radiometer output after the exclusion of its constant component. Algorithm bases on two operations. These operations are performed on the signals after the low-frequency gain: the exclusion of constant component and the definition of the voltage polarity in the second half-cycle modulation in which pulse-width modulation is not performed (synchronous comparison with zero potential). An application of zero receiving method modification in radiometers leads to a considerable construction simplification and measurement accuracy improvement. An indicator of the zero balance in the radiometer is an equality to zero of voltage in the second half-period of the symmetric pulse modulation at the radiometer output.
An algorithm of radiometer follow-up system functioning was developed on the base of combined pulse modulation and the original principle of the signal processing. This principle showed that the auto-zero balance can be carried out by changing of the pulse-width signal duration. As a result a transfer characteristic was obtained. This characteristic mathematically describes the work of the radiometer with combined pulse modulation and zero receiving. According to this characteristic the noise antenna signal is determined by the reference signal duration modulated by pulse-width-law without any additional form transformations of modulated signal sequences in the low-frequency path.
An integrated block diagram of a modified zero-radiometer was created. This diagram is the base diagram for the design of a new class of portable radiometric systems. In this scheme the receiver input block defines the radiometer basic characteristics: fluctuation sensitivity, temperature and time stability, measurement range. The problems of the designing of the input blocks follow diagrams were considered. Three structural models were proposed and selected as the reference. Radiometers with optimal parameters are creating by using of these models. Optimization is performed by the minimum detection threshold of antenna signal, by the using noise generator, the design philosophy.
A formula for the fluctuation sensitivity calculating of the modified zero radiometers was obtained in general terms. The analysis of the fluctuation sensitivity of this measurement method indicated on the variable character of the fluctuation and the depen-dence of the measured antenna signal. Formulas for calculating the fluctuation sensitivity for three proposed input blocks schemes of the modified zero-radiometer were obtained and experimental verification was carried out. These three schemes are differenced by the range of measured signals.
The calculating method of the parameters of low-frequency processing path depending on the required threshold of signal detection was obtained. The identification of significant digits number of radiometer output digital code was received. The receiving data are sufficient for the design of the modified radiometer, which has in structure basic input block.

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